diff --git a/libisofs/libisofs.h b/libisofs/libisofs.h
index 70c6dd7..721f347 100644
--- a/libisofs/libisofs.h
+++ b/libisofs/libisofs.h
@@ -878,15 +878,17 @@ extern ino_t serial_id;
 struct IsoStream_Iface
 {
     /*
-     * Current version of the interface, set to 1 or 2.
+     * Current version of the interface.
      * Version 0 (since 0.6.4)
      *    deprecated but still valid.
      * Version 1 (since 0.6.8) 
      *    update_size() added.
      * Version 2 (since 0.6.18)
-     *    get_input_stream() added. A filter stream must have version 2.
+     *    get_input_stream() added.
+     *    A filter stream must have version 2 at least.
      * Version 3 (since 0.6.20)
-     *    compare() added. A filter stream should have version 3.
+     *    compare() added.
+     *    A filter stream should have version 3 at least.
      * Version 4 (since 1.0.2)
      *    clone_stream() added.
      */
diff --git a/libisofs/stream.c b/libisofs/stream.c
index bb9806a..551778f 100644
--- a/libisofs/stream.c
+++ b/libisofs/stream.c
@@ -214,6 +214,7 @@ int fsrc_clone_stream(IsoStream *old_stream, IsoStream **new_stream,
     return ISO_SUCCESS;
 }
 
+static
 IsoStreamIface fsrc_stream_class = {
     4, /* version */
     "fsrc",
@@ -503,6 +504,7 @@ int cut_out_clone_stream(IsoStream *old_stream, IsoStream **new_stream,
 /*
  * TODO update cut out streams to deal with update_size(). Seems hard.
  */
+static
 IsoStreamIface cut_out_stream_class = {
     4, /* version */
     "cout",
@@ -748,6 +750,7 @@ int mem_clone_stream(IsoStream *old_stream, IsoStream **new_stream,
 }
 
 
+static
 IsoStreamIface mem_stream_class = {
     4, /* version */
     "mem ",